KH 15 D: Now You See It, Now You Don't

Published: Summer 2004

It’s a neat trick. Every 48 days a recently discovered speck of light in the night that astronomers call KH 15 D vanishes. Then, 20 days later —presto—the star reappears. What’s going on? As with any disappearing act, it’s most likely smoke and mirrors, or, in this case, the cosmic equivalent.

Peter Garnavich, a Notre Dame associate professor of physics, and colleagues from the Harvard-Smithsonian Center for Astrophysics and Wesleyan University theorize that a large, swirling disk of dust and gas, “leftovers” from the star’s birth which eventually will coalesce into a planet, is the cause of the star’s periodic winking.

Perhaps most significantly, Garnavich and his associates found that KH 15 D’s long, periodic eclipse is a new phenomenon; the star didn’t always wink. The physicists discovered no evidence for the eclipse when they analyzed photographic plates from the Harvard photographic plate archives of the night sky from the first half of the 20th century.

“There are very few cases where astronomers can see a significant change to a star over a single human lifetime,” Harvard researcher Joshua Winn says. “And if the eclipses are caused by material in a protoplanetary disk, as suspected, then that would give us the exciting opportunity to study planet formation on surprisingly short time scales.”

By studying when and how the eclipses begin the scientists hope to learn more about KH 15 D and the planet forming around it.